1. Field of the Invention
The present invention relates to a printed wiring board and a method of manufacturing the printed wiring board and, more particularly, to a printed wiring board having a fine wiring pattern of high-density wiring design and a method of manufacturing the printed wiring board.
2. Description of the Related Art
Methods of forming a circuit in a printed wiring board are divided into the subtractive method which involves forming an etching resist pattern on metal foil, such as copper foil, and forming a wiring pattern by etching the part of metal foil not covered by the etching resist pattern; and the additive method which involves forming a plating resist of a pattern reverse to a circuit and forming a wiring circuit by depositing a plating on an opening of this plating resist.
Because the manufacturing steps in the subtractive method are easier than in the additive method, it is possible to manufacture circuits at very low cost. However, in the plating and an electrolytic plating to the whole insulating substrate when forming through-holes and blind-via-holes etc. and, therefore, the thickness of the conductor layer (metal foil plus a plating) to be etched becomes very large. Therefore, it was difficult to form a good circuit. Particularly, the subtractive method was not suitable for the formation of a fine wiring pattern having pattern line/pattern space of not more than 75 μm/75 μm.
In contrast, the additive method, which is favorable for the formation of a fine wiring pattern, requires that a wiring pattern be formed by depositing a plating on an insulating layer and, therefore, compared to a case as in the subtractive method where an insulating substrate in which metal foil has been laminated on an insulating layer is processed, the additive method had problems such that the adhesion of the wiring pattern is poor.
Furthermore, in terms of the design of printed wiring patterns, wiring patterns are formed nonuniformly in the plane of the substrate and, therefore, when wiring patterns are formed by selective plating as in the additive method, currents are concentrated more than necessary in portions of coarse wiring patterns and variations in the thickness of the wiring pattern become large, resulting in problems such that it is difficult to ensure impedance matching.
To solve such problems, the present inventors have invented a method of manufacturing a printed wiring board of a construction as shown in FIGS. 6(a) to 6(c) (FIGS. 6(a) to 6(c) show a condition without a positional deviation in the laser beam machining and exposure steps), and the patent application of this invention has already been released to the public (refer to JP-A-2004-319994).
FIGS. 6(a) to 6(c) show an example in which buildup wiring layers are formed on an inner-layer core substrate, which is not shown. First, as shown in FIG. 6(a), an insulating layer 1 and metal foil 2, such as copper foil, are laminated in this order on a layer on which a lower-layer wiring pattern 3 is formed, which is formed in an inner layer (for example, resin-including copper foil is laminated). Subsequently, a nonthrough hole 7 which reaches the lower-layer wiring pattern 3 is made by laser irradiation (refer to FIG. 6(b)).
Next, after desmear treatment of the nonthrough hole 7 is performed, as shown in FIG. 6(c), a barrier metal layer 6 (for example, Ni—B and Ni—P) is formed by substitutional nonelectrolytic plating treatment on the area of the surface of the lower-layer wiring pattern 3 uncovered by the metal foil 2 and the nonthrough hole 7.
Next, a nonelectrolytic plating (for example, a nonelectrolytic copper plating), which is not shown, was formed on the whole area of an outer layer including the nonthrough hole 7. Subsequently, by performing electrolytic plating treatment (for example, electrolytic copper plating treatment using a plating liquid for filled vias), a plating 8 is filled in the nonthrough hole 7 and the plating 8 is deposited also on the outer layer (refer to FIG. 6(d)).
Next, as shown in FIG. 6(e), after the pattern formation of a blind via hole 10 and its land portion (hereinafter referred to as a “plating land 8a,”), as shown in FIG. 6(f), the barrier metal layer 6 exposed to the outer layer is removed by etching and subsequently, circuit is formed on the exposed metal foil 2, whereby a printed wiring board Pa of FIG. 6(g), on the outer layer of which an upper-layer wiring pattern 4 is formed is obtained.
Because a plating is deposited also on the outer layer when a plating is filled in a blind via hole as described above, compared to a case where a plating is deposited only in the formation portion of blind via hole, the blind via hole can be filled with the plating in a more stable manner (and it is possible to eliminate variations in the amount of deposited plating caused by either coarse or dense condition of the area on which the plating is deposited) and by forming the wiring pattern by etching the metal foil which has been laminated on an insulating layer, it is also possible to ensure the adhesion of the wiring pattern to the insulating layer. Furthermore, it is possible to keep the thickness constant because the metal foil is protected by the barrier metal layer and, therefore, a fine wiring pattern (equivalent to the upper-layer wiring pattern 4 in the figure) can be easily formed.
However, in the above-described construction, processing involves three steps in total, i.e., laser beam machining when the nonthrough hole 7 is made, and the exposure steps in the formation of the plating land 8a and the metal foil land 2a (the outermost land obtained by performing the circuit formation of the metal foil 2 shown in FIG. 6(g)). Therefore, when all alignment accuracy is considered, the land diameter (equivalent to the above-described metal foil land 2a) becomes considerably large, thereby impairing high-density wiring design (this applies also to a manufacturing method which involves etching a window portion for laser beam machining in metal foil beforehand and then making a nonthrough hole).
Simply, because laser beam machining is performed directly from above the metal foil 2, in terms of the machining accuracy of a laser beam machine, a positional deviation occurs in the range of about 20 μm one side (40 μm in diameter) or so from the reference point and, therefore, in a case where the plating land 8a is formed by etching the plating 8 later, it is necessary to consider the above-described amount of positional deviation.
More specifically, FIG. 7(a) is a plan view which shows the nonthrough hole 7 made in the step of FIG. 6(b) as viewed from above and if it is supposed that the nonthrough hole 7 is in the reference position, then it follows that a positional deviation occurs in the range indicated by a dotted line (in the range in which the diameter is 40 μm larger than the diameter L1 of the nonthrough hole 7).
Therefore, in order to form the plating land 8a so that the nonthrough hole 7 does not run off the plating land 8a, when the exposure accuracy during the formation of an etching resist pattern, which is not shown, (for example, a positional deviation in the range of about 20 μm one side (40 μm in diameter) from the reference point) is also considered, as shown in FIG. 7(b) (a plan view obtained when FIG. 6(e) is viewed from above), it is necessary to form the plating land 8a with a diameter L2 which is larger than the diameter of the nonthrough hole 7 by 40 μm one side (diameter 80 μm) and hence it was necessary to set the diameter L3 of the outermost metal foil land 2a at a very large value. Incidentally, FIG. 7(c) (a plan view obtained when FIG. 6(g) is viewed from above) shows a case where the plating land 8a is formed so that the plating land 8a does not run off the metal foil land 2a and also in this case, the exposure accuracy must be considered. Therefore, the diameter L3 of the metal foil land 2a becomes larger than the diameter of the nonthrough hole 7 by 80 μm one side (160 μm in diameter).
The present invention has been made to solve the above problem and has as its object the provision of a printed wiring board having a fine wiring pattern in which high-density wiring design is aimed at by eliminating a positional deviation between a blind via hole and a land, and a method of manufacturing this printed wiring board.